Of the files mentioned above, note that PF has radically changed since OpenBSD 3.7 -- to the point where of essentially being completely rewritten. Whatever pf.conf you have now in 3.7 is likely to incompatible, however, you can use it as the basis to rewriting in 4.6. As for the other files mentioned, I don't recall that there has been changes to resolv.conf. There are also the password file, configurations to DHCP if you are running the server, & any other daemon which you may have customized.

Note that you can upgrade from 3.7, but you will need to upgrade incrementally from 3.7 t o3.8, 3.8 to 3.9, etc.. all the way up to 4.6 Personally, I would copy anything of value, & simply reinstall given the laborious process of updating successive versions, but what you do is your choice.

Honestly, you shouldn't let systems run unmaintained for so many years.. they need to be a priority.. releases happen every 6 months, security/reliability updates are made available on the errata page.

All-in-all, a release is supported by the developers for 12 months.. plenty of time to prepare a decent upgrade policy.